Touchstone (assaying tool)
[file:Pierre de touche.jpg|thumb|Touchstone set]
A touchstone is a small tablet of dark stone such as slate or lydite, used for assaying precious metal alloys. It has a finely grained surface on which soft metals leave a visible trace. Basic requirements for a touchstone: hardness on the mineralogical scale 4.6-6.5, the touchstone must be free of cracks and not react with inorganic acids and their mixtures.
History
The touchstone was used during the Harappa period of the Indus Valley civilization ca. 2600–1900 BC for testing the purity of soft metals. It was also used in Ancient Greece.The touchstone allowed anyone to easily and quickly determine the purity of a metal sample. This, in turn, led to the widespread adoption of gold as a standard of exchange. Although mixing gold with less expensive materials was common in coinage, using a touchstone one could easily determine the quantity of gold in the coin, and thereby calculate its intrinsic worth.